The MRI apparatus is an apparatus that measures an NMR signal generated by an object, especially, the spin of nuclei that form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In imaging, an object is disposed in a static magnetic field (polarized magnetic field B0), and then a high frequency magnetic field pulse is applied together with a slice selection gradient magnetic field pulse in order to selectively excite a specific region. Then, a phase encoding gradient magnetic field pulse or a readout gradient magnetic field pulse is applied for encoding within the excitation range, thereby giving position information.
As for the gradient magnetic field pulse, a desired gradient magnetic field pulse (referred to as an output gradient magnetic field waveform) is output by supplying a current changing in a pulse shape (referred to as an input gradient magnetic field waveform) from a gradient magnetic field power source to a plurality of coils that generates linear gradient magnetic fields in directions of three axes perpendicular to each other. Ideally, an input gradient magnetic field waveform and a gradient magnetic field waveform generated in the MRI apparatus by the input gradient magnetic field waveform should be equal. However, error is caused by various factors.
One of the causes of the error is the influence of the eddy current generated in a magnetic component or an electrical circuit, which forms the MRI apparatus, due to rapid changes in the magnetic field due to the gradient magnetic field pulse. The eddy current causes a change in the magnetic field in the opposite direction to a change in the magnetic field due to the gradient magnetic field pulse depending on time and space, distorting the output gradient magnetic field waveform.
In addition, generally, the output gradient magnetic field waveform of the gradient magnetic field coil includes distortion such as a response delay depending on the characteristics (Q value) of the gradient magnetic field coil. In order to compensate for this, a circuit that performs feedback control and the like is included in a gradient magnetic field generation system including a gradient magnetic field power source in many cases. Such a control circuit plays a role of compensating for the eddy current, but may also be a cause of generating an output gradient magnetic field waveform that is different from the input gradient magnetic field waveform.
Distortion of the output gradient magnetic field waveform caused by the influence of the eddy current and the control circuit of the gradient magnetic field power source changes depending on the shape of the input gradient magnetic field waveform, and various problems occur depending on the application axis of the gradient magnetic field.
For example, distortion of the slice selection gradient magnetic field pulse causes an error in the excitation profile and the excitation position. In particular, the influence of this distortion in the case of a VERSE (variable rate selective excitation) method of applying the high frequency magnetic field pulse while changing the application strength of the gradient magnetic field pulse is large.
Distortion of the readout gradient magnetic field pulse causes distortion or artifacts, such as ghosting, in an image. In particular, the influence of this distortion is noticeable in cases of Echo Planar Imaging (EPI) measurement in which measurement is performed while reversing the application polarity of the readout gradient magnetic field pulse, spiral measurement to scan k-space spirally, ultra-short TE measurement to start scanning from the k-space center, and the like.
In order to solve the above-described problems, it is necessary to compensate for the distortion of the output gradient magnetic field waveform that changes depending on the input gradient magnetic field waveform. As known compensation techniques, methods shown below have been proposed.
One method is to apply a magnetic field for compensating for the eddy current using a shim coil, a gradient magnetic field coil, and the like. This method is to compensate for the distortion of the output gradient magnetic field waveform by measuring a magnetic field, which is caused by the eddy current generated after applying a gradient magnetic field pulse, temporally and spatially and outputting the waveform of a gradient magnetic field pulse to cancel the output characteristics of the eddy current obtained from the measurement result (PTL 1 and PTL 2).
In addition, as a method dedicated to compensating for the distortion of the slice selection gradient magnetic field pulse, a method of changing the irradiation timing of the high frequency magnetic field pulse according to the output gradient magnetic field waveform has been proposed (PTL 3). In this method, degradation of the excitation profile is suppressed by measuring the distortion of the gradient magnetic field pulse, calculating the delay time from the center of gravity of the pulse area, and changing the timing of the high frequency magnetic field pulse and the gradient magnetic field pulse on the basis of the delay time. In addition, a method of eliminating a change in the output gradient magnetic field waveform according to the input gradient magnetic field waveform by using a fixed input gradient magnetic field waveform has also been proposed (NPL 1). In this method, when changing the excitation width of the high frequency magnetic field pulse, the amplitude of the high frequency magnetic field pulse is adjusted instead of changing the input gradient magnetic field waveform.
As a method dedicated to compensating for the distortion of the readout gradient magnetic field pulse, a method of modeling a system response using an RLC circuit, estimating an output gradient magnetic field waveform for an input gradient magnetic field waveform, calculating the coordinate position of the NMR signal in k-space using the result, and compensating for image distortion has been proposed (NPL 2). In this method, coefficients of the modeled expression are determined on the basis of the appearance of the image. In addition, instead of the modeling of the method described above, a method of compensating for image distortion by calculating the coordinate position of the NMR signal in k-space from the result obtained by measuring the gradient magnetic field waveform after the end of imaging has also been proposed (NPL 3).